The objective of the present study is to chart the relative positions of motoneuron nuclei supplying individual muscles, or small groups of synergists, in the forelimb of the cat using the retrograde transport of the enzyme, horseradish peroxidase (HRP), applied to cut ends of peripheral nerve bundles. Using TMB as the chomogen to reveal the presence of HRP and special proceedures to prevent the uptake of HRP in nerves other than the desired one leads to essentially a reliably complete labelling of alpha and gamma motoneurons innervating a muscle thereby permitting accurate counts of cell populations. The same labelling method has been found to be transported in an anterograde fashion, transganglionically, to reveal zones of termination of primary afferent fibers within the dorsal horn of the spinal cord and dorsal column nuclei of the brain stem. By judicious selection and preparation of identified peripheral nerve branches it is possible to reconstruct the relative positions of fields of termination of the central processes of primary afferent fibers from spatially restricted peripheral receptive fields from both skin and muscle tissue. Pilot studies have shown that large diameter afferents from skin and muscle terminate in distinctly different laminae of the dorsal horn and that the positions of the afferent terminal populations differ according to the longitudinal location relative to the spinal segments which receive the sensory fibers. The separation of muscle and cutaneous afferent central projection is also visible at the level of the dorsal column nuclei. Systematic reconstruction of the anterograde and retrograde distribution of HRP from anatomical subdivisions of the peripheral nervous system can be used to chart out the central topography of the origins and terminations of fibers within a given peripheral nerve. Eventually, it will be possible to develop a complete topography of spinal cord and brain stem cell populations involved in motor and sensory functions of the brachial plexus.